Sorry for the delay replying. It's early stages for me but I plan on picking up the entire scupper kit from somewhere like this Whale Marine - Products
IIRC it'll sense waste water in the drain, pump it up and over to the grey tank (top inlet) and then shut off the pump. I did a bit of research and as with everything some love them and some hate them. I plan for either a slim space between the tub and floor or maybe under the floor with an access hatch in the belly so I can service the kit. (or just replace it when it fails). If it doesn't have a back flow valve I'll add one if possible.
On my previous Airstream build I had a 4" platform under the tub and it caused me no problems but I am leaning towards the on floor tub this time for added roominess as I'll be using the original tub which is situated further into the end cap radius .